PP mutexes: unlock: Reset inherited prio regardless of privileges

'uq_inherited_pri' contains the current priority inherited from Priority
Protection mutexes. If -1 is passed through 'm_ceilings[1]', meaning
that there are no such mutexes held anymore, this must be reflected into
it by setting it to PRI_MAX, regardless of whether the thread has
privilege to set realtime priorities (PRI_MAX is also obviously not
a realtime priority level). By contrast, it shall not be updated and
the computed 'new_inherited_pri' shall stay unused if the thread doesn't
have the ability to set a realtime priority, possibly keeping an older
such priority acquired previously.

Reviewed by: kib
Approved by: emaste (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43984